This mentored research training proposal provides the necessary time and training to augment and enhance new research into how defective Ca2+ homeostasis and signaling may be involved in hearing loss. Mentored research training will occur in the in vivo assessment of cochlear function within the laboratory of Dr. M. Charles Liberman (Aim 1); in the measurement of hair cell motility within the laboratory of Dr. David He (Aim 2); and in the measurement of hair cell cytosolic Ca2+ levels within the laboratory of Dr. Fabio Mammano (Aim 3). Our preliminary data suggest that targeted disruption of oncomodulin (Ocm), a major Ca2+ binding protein mostly in outer hair cells (OHCs), leads to progressive hearing loss. We hypothesize that Ocm regulates cyto- solic Ca2+ levels necessary to protect OHCs from noise damage and aging defects. Specifically, this proposal investigates mechanisms that in the absence of Ocm might make the ear more vulnerable to noise and aging. Specific Aim 1 tests the hypothesis that Ca2+ buffering by Ocm alters efferent-mediated responses. First, we will test in vivo OHC function in Ocm mutants by measuring DPOAE thresholds and growth curves and measuring cochlear potentials, with a focus on the cochlear microphonic (CM) potential. DPOAEs provide a window into cochlear amplification. Although dependent on frequency and SPL, the CM response can provide a measure of the transducer capability of OHCs and is an indication of hair bundle functionality. Second, we will test if a lac of Ocm alters efferent-mediated responses. Efferent stimulation normally decreases the OHC contribution to cochlear amplification, thereby eliciting fast suppression of DPOAE amplitudes. Specific Aim 2 tests the hypothesis that Ca2+ buffering by Ocm alters Ca2+-dependent motile responses of OHCs.0Increases in cytosolic Ca2+ levels lead to OHC elongation mediated by Ca2+-dependent phosphoryla- tion. However, the mechanisms that induce OHC shortening or a decrease in axial stiffness are little understood. We will investigate OHC motility and electrophysiological responses. We will measure length changes of OHCs to voltage steps in whole cell, voltage clamp conditions. To assess whether targeted deletion of Ocm affects voltage-gated channels, we will also measure OHC current-voltage relations and capacitance. Specific Aim 3 tests the hypothesis that Ca2+ buffering by Ocm alters the size of Ca2+ transients. Defects in Ca2+ regulation could lead either to a broadening of Ca2+ transients or increases in the amplitude of Ca2+ signals. We will investigate induced [Ca2+]i flux using fluorescent-based measurements. We will also compare influx of extracellular Ca2+ to internal (store) Ca2+ in Ocm mutants. In summary, the laboratories chosen for research training will provide the PI with new strategies and tools to employ within his own laboratory to further understand the role of Ca2+ regulation in deafness and enhance competitiveness for future funding.